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Degradation and recovery of polydimethylsiloxane (PDMS) based composites used as high voltage insulators

Abraham Berhane, Teclesenbet (2004-12)

Thesis (MSc)--Stellenbosch University, 2004.

Thesis

ENGLISH ABSTRACT: Polydimethylsiloxane (PDMS) compounds are utilized in outdoor high voltage insulation
due to their low weight, vandalism resistance, better anti-contamination performance
and their superior hydrophobic nature. Under severe environmental conditions and over
prolonged service time, however, the hydrophobic surface can gradually become
hydrophilic and then recover with adequate resting period.
In this study, room temperature vulcanized (RTV) PDMS samples were prepared with
different formulations and then exposed to corona discharge to evaluate its effect. The
influence of different additives, such as different types and amount of fillers and
additionally added low molar mass silicone oils, on the hydrophobicity recovery of the
material was investigated. The effects of two types of corona treatment were also
evaluated.
Hydrophobicity recovery of corona and UV-C aged PDMS samples was evaluated by
means of static contact angle measurements.
Positron annihilation spectroscopy (PAS) gave important information on the micro
structural change after corona treatment of RTV PDMS as well as naturally aged high
temperature vulcanized (HTV) PDMS samples. The different formulations of the RTV
PDMS samples and the effect of the additives were studied with this technique. The
formation of a thin, highly crosslinked inorganic silica-like (SiOx) layer was confirmed
even at the early stage of degradation. It was also possible to estimate the thickness of
the silica-like layer formed during corona exposure that is responsible for the loss and
recovery of hydrophobicity.
The surface hardness and hydrophilicity change of PDMS samples due to corona
treatment were studied simultaneously with force distance measurements by atomic
force microscopy (AFM). The adhesive force calculated from the pull-off force-distance
curves showed that the adhesive force between the probe and the sample decreased
with increasing corona treatment time, indicating hydrophobicity recovery. In addition to
this, the increase in hardness after corona exposure provides indirect evidence of the
formation of a silica-like layer. In all cases the hydrophilicity and the surface hardness of
the PDMS samples increased directly after corona treatment and recovered with time.
Two types of FTIR spectroscopy were used to analyse the surface of the polymer.